This invention relates generally to the detection of explosive gases in a gaseous medium such as air.
In a manner known in itself, the the detection of explosive gas in air is performed in an apparatus usually called catalytic filament explosimeter in which a filament generally of platinum is heated by Joule effect i.e. by the passage of an electric current therein. The explosive gas contained in air oxidizes by catalysis upon contacting the filament, thereby causing additional heating of the latter. The increase in temperature resulting therefrom causes increased resistance of the filament and measurement of such resistance gives access to the concentration of such explosive gas in air. Practically, it is dealt with the gas explosivity level i.e. the ratio of its concentration to its lower limit of explosiveness (LIE), i.e. the gas content above which there is a risk of explosion; this is why the detection result is usually presented as a percentage of the mentioned limit LIE.
By way of example the Applicant has proposed in its U.S. Pat. No. 3.449.939 a portable gas sampling and metering-in-air device. Gas meter;ng is effected in the conventional manner by measurement of the voltage across one diagonal of a resistive bridge consisting of e detecting filament and a compensating filament mounted in parallel connection with two resistors, one of which is advantageously adjustable, and the bridge being supplied with electric power along another diagonal.
The Applicant has also proposed in a more general scope in its U.S. Pat. No. 3.587.318 a method and apparatus for measurement of a characteristic quantity of a gaseous medium according to which the value of the quantity considered is appreciated from the measurement of one of the supply power data of a detecting filament where the resistance thereof is kept equal to that of a compensating filament.
In use such explosimeters turn out to give very good results for the explosive gas which was used for the rating thereof whereas with other gases the estimated concentration rates largely deviate from the actual ones.
Such deviations partly result from the differences between the gases, in particular as regards their oxidization heat, combustion temperature, thermal conductivity and above all their diffusion coefficient in air (for example, hydrogen diffuses four times quicker than methane). Such differences influence the renewal of explosive gas at the filament, the quality of combustion (incomplete or premature depending on the filament temperature), the number of calories thus available and their discharge into the surrounding gaseous medium. It is to be noted that such deviations often correspond to underestimation of the actual values, and that this is detrimental to security.